Title page for ETD etd-05312012-212545

A 2-D, Variable-Density Numerical Model of Subsurface Fluid Flow through the Edwards Aquifer, New Braunfels, TX: Mechanisms Inhibiting Flow across the Freshwater/Saline-Water Interface

Degree

Master of Science (M.S.)

Department

Geology & Geophysics

Advisory Committee

Advisor Name

Title

Wicks, Carol

Committee Chair

Hanor, Jeffrey

Committee Member

Nunn, Jeffrey

Committee Member

Keywords

Edwards Aquifer

fluid flow

saline

saltwater

intrusion

balcones fault zone

karst

model

Date of Defense

2012-05-07

Availability

unrestricted

Abstract

The Edwards aquifer in south-central Texas, U.S., composed of faulted carbonate bedrock, contains freshwater and saline water. In aquifers that are used as a source of drinking water and that contain fresh and saline waters, saline-water intrusion can result in degradation of water quality. Yet, in the Edwards aquifer, limited saline-water intrusion has occurred. The questions addressed are “Why is the saline-water intrusion less than expected,” and “Is there a trigger that will result in saline-water intrusion into the freshwater reservoir?” Three hypotheses were tested. One: an extremely saline water density might prevent mixing across the interface. Two: faults could be acting as a barrier between the freshwater and saline-water zones, preventing movement of the saline water into the freshwater zone. Three: the permeability of the bedrock in the saline water zone might be extremely low, limiting movement of the saline water. A transect of observation wells was chosen in New Braunfels, TX for study.

2-D, variable-density numerical models of groundwater flow were used to determine which factor controlled the lack of saline-water intrusion. Numerical models were produced for each hypothesis using Basin2. It is clear from each model that fault permeability and fault compartmentalization is the primary mechanism inhibiting flow across the freshwater/saline-water interface. When horizontal fault permeability reached 0.01 D, flow was significantly reduced across the interface. When these values reached 0.001 D, flow across the interface ceased. To a lesser extent, saline-water zone permeability controlled the movement of flow across the interface, if permeability values were reduced by three orders of magnitude. However, extremely high saline water densities did not inhibit flow.

A trigger that would increase fault permeability would be continued dissolution of the carbonate rocks, but it most likely would take tens of thousands of years for dissolution to significantly increase the permeability of the fault surfaces. In addition, dedolomitization of the saline water zone would be another trigger that would induce flow across the interface, but since little diagenesis in the saline water zone has been observed, dedolomitization is not a pressing issue.